A hydraulic brake booster is coupled to a pressure source and is selectively actuated by an operator to provide a power assist to braking. The booster housing supports a piston which is movable in response to pressure in a pressure chamber and carries a control valve which is movable by an actuator to control fluid communication from the pressure source to the pressure chamber via an inlet and an outlet on the housing.
In order to control the amount, or the pressure level, of the pressurized fluid communicated to the pressure chamber, a pressure limiting device, such as illustrated in U.S. Pat. Nos. 3,724,332 and 3,728,942, is provided to relieve pressure in the pressure chamber when the pressure therein is above a predetermined value.